1. DMD is an X-linked recessive trait, meaning the gene is located on the X chromosome and the disorder is expressed in males who inherit the mutant allele on their single X chromosome.
2. Individual I-1 is a male (square) and does not have DMD, so his genotype is X^Y (normal X chromosome and Y chromosome).
3. Individual I-2 is a female (circle) and has DMD, so her genotype must be X^X^ (homozygous recessive, as she expresses the disorder).
4. Since I-1 has a normal X chromosome (X^), he can only pass on his X^ chromosome to his daughters and his Y chromosome to his sons.
5. I-2, being affected, has two X^ chromosomes and will pass one of them to each child.
6. If I-1 and I-2 have another son, the son will inherit the Y chromosome from I-1 and one X chromosome from I-2.
7. Since I-2 has two X^ chromosomes, the son will definitely inherit an X^ chromosome from her.
8. Therefore, the son will have the genotype X^Y, which means he will have DMD.
9. The chance that the son would have DMD is 100%.
Answer: 100%
Parent Tip: Review the logic above to help your child master the concept of pedigree practice worksheet.
